It is know from U.S. Pat. No. 3,789,612 is a method that makes waves in a volume of water.
In one of the options of this method, a massive plunger, to which a vertical reciprocating motion is applied, is used for making waves. The plunger requires significant mechanical means to move it and to guide it.
In another option of that method, an open lower end tank is used. When the tank is lifted up above the surface of the water, the water volume that it contains discharges suddenly, resulting in the creation of a disturbance. This system allows the creation of one wave, but not of a system of sustained waves.
In international application WO 91/14062 a motion generation system in a liquid is known, especially at the surface of that liquid. That system uses a device containing two elements one of which moves relatively to the other. That relative displacement creates inertial forces, which make the whole device move in the liquid. That displacement creates excitations in the liquid, which are transformed in waves. In one of the embodiments of that system, one of the elements is a hollow sphere in which the second element moves. That kind of system effectively allows making waves at the surface of a liquid. But it has several disadvantages.
In the embodiment where the device is a sphere and also in other forms, the vertical stability of the system is not satisfactory. Specially, it can turn upside down when the mobile mass is in high position with respect to the shell.
The inertia forces being proportional to the moving mass, the mass of the device must be significant to create appreciable waves. The weight of the device makes its handling difficult. Moreover, the components designed to make the relative movement of the two elements have to be dimensioned accordingly, to be able to displace that significant mass, which has an incidence on the cost.
When a mobile element is moved by a harmonic movement of fixed amplitude, for example by a crank-connected rod system, the inertia force created is proportional to the weight and to the square of the frequency of the movement. It is thus not possible to adjust that excitation force independently from the frequency, or conversely, it is not possible to modify the frequency without modifying the excitation force. At the start, it is not possible to progressively increase the excitation force before reaching normal operating conditions.
The components designed to make the relative movement are part of the device. In that way, they are under the ambient conditions of the liquid, and are subject to damage by any small tightness default of the box. Moreover, they are hardly accessible for maintenance.
In one form of embodiment of that device, the power needed to put in motion the elements of the device is transferred to the device, for example by the means of an electrical cable. That cable is moved by the repetitive movements of the device. It is subject to wear or may be broken, and represent a risk. Moreover, the presence of that cable in the liquid makes a constraint.
In another form of embodiment of that device, the power needed to put the elements of the device in motion is stored inside the device, for example by means of electric energy in a battery, or pressure energy in a tank. In that case, the periodical need of recharge is a disadvantage for its operation.
Different wave machines based on various principles, for example by the EP 0 236 653, or EP 0 732 468, are also known but all request a major adaptation of the pool. These devices are not adapted to the generation of waves in existing pools.